Sea cucumbers are marine animals from the phylum echinoderm and class Holothuroidea. Of the more than 1250 species worldwide, many are gathered for human consumption or grown in aquaculture systems. Like other echinoderms, sea cucumbers have pentagonal radial symmetry. But unlike starfish, they are oriented with soft, cylindrical bodies like they are lying on their sides. Sea cucumbers have a leathery skin covering an endoskeleton of calcified structures of isolated microscopic ossicles (or sclerietes) joined by connective tissue and five longitudinal muscle bands. The body walls of sea cucumbers are formed of catch collagen fibers, which can be loosened and tightened by the animals, permitted them to either form a hard endoskeleton through their dermis layer or effectively liquefy their connective tissue and pour themselves through small openings and then reconnect the collagen into a firm consistency on the other side. The mouth is located at the oral end, identified by a circle of branching tentacles about it, and the anus opens at the aboral end, defining an aboral/oral axis. The internal organs (viscera) of the sea cucumber lay within the tube-like body chamber also referred to as the coelomic cavity. Certain of the species also discharge the toxic chemical holothurin (named for the class of species) as a defense mechanism when startled.
They range in length from less than 1 centimeter to almost a meter (though most are between 10 cm and 30 cm in length) and in thickness up to 30 cm.
The great variations in size and shape, and the animal's defensive techniques, make it difficult to process either by hand or machine—even within a single species.